1. Field of the Invention
This invention relates to sealable multimetallic tube sleeves and methods of installing these sleeves in degraded nuclear steam generator tubes, and the like.
2. Description of the Prior Art
The economic performance of any power plant is closely related to the availability of the plant. In the nuclear industry, operating experience has demonstrated that the steam generator is a critical component of the plant in maintaining high availability. Steam generator tube leaks affect short term availability through forced outages. In addition, profilometer or eddy current examination of tubing and subsequent plugging of degraded or failed tubes will extend outages. However, this is a minor effect on availability compared to the influence of running at reduced power for extended periods of time, of derating the plant due to insufficient heating surface caused by tube plugging or the complete replacement of the steam generator.
Nuclear steam generators generally fall into two classes: the once-through design with vertically extending straight tubes and upper and lower tube sheets, and the U-tube design with a lower tube sheet from which vertically extending tubes are secured after a 180 degree turn in the upper region of the steam generator. Due to the close spacing of the tubes and operating pressure, ligament efficiency is low requiring tube sheet thicknesses in the order of 24 inches. In the initial assembly of the steam generator, the tubes are expanded for about 3 inches at the lower ends and seal welded to the lower face of the lower tube sheet. A crevice of about 7 mils exists above the expanded portion of the tubes to the top face of the tube sheet. This crevice and the region immediately above the top face of the tube sheet is the site of caustic intergranular attack and stress corrosion of the tubes in operating units. Plugging the tubes obviously removes heating surface and sleeving, heretofore used, is only a temporary expedient since the sleeves will be subject to the same destructive environment as the tube. The problem to be solved is to identify a material suitable for a long-time repair of the degraded tube. This material must meet several requirements: it must have resistiveness to the corrosive environment that caused the original tube to fail, it must have physical and mechanical properties equivalent to those of the original tube, and it must also have the same corrosion resistance on its interior surface exposed to the primary side (tube inside diameter) environment of the heat exchanger as the original tube.
Clearly, there is an urgent need for an improved solution for repairing degraded or leaking steam generator tubes.